The present invention relates to a waxless ski base of a plastic composite material and methods of making such a base.
Plastic ski base materials generally have satisfactory glide properties against snow irrespective of snow conditions. These properties are in fact optimally utilized on alpine and jumping skis. However, the excellent gliding properties of plastic ski base materials against snow represents a significant problem when used on skis of the cross country or touring type, since it is a requirement for such skis that they do not slip or slide backwards in the kick off phase of the skier's movements, particularly when climbing a slope.
Up to the present time, two different procedures have been employed in order to avoid slip or back sliding of glassfibre armed skis with a plastic base of the cross country or touring type. One procedure is to obtain the necessary grip properties by use of a suitable ski-wax. The other procedure involves the provision of a no-wax plastic base, i.e. a base of such a nature or configuration that the desired frictional properties are obtained without the need for waxing the gliding surface.
Attempts have been made to provide improved wax-adhering plastic bases by imitating the porous fibre structure of the wooden ski by adding various filling components, such as randomly arranged fibres of a textile material. Further, a plastic base has been suggested which comprises a matrix having particles embedded therein which may be aligned in certain directions, for instance longitudinally of the ski. The particles, which may be in the form of fibres or flakes, are supposed to be soluble in water. When using such skis, wear will expose the particles which then dissolve and leave open pores in the sole to improve adherence of ski wax.
In ancient times no-wax ski were obtained by attaching fur strips to the mid portion of the ski base. In modern skiing this principle has been utilized for example by using a plush like fabric in a plastic matrix, as disclosed in Austrian patent No. 317,734, the fabric fibres and the matrix combining into a composite material forming the base. During wear of such base materials a three-dimensional patterned ski-profile is formed, the fibres protruding slightly from the matrix and presenting a patterned micro structure which also facilitates waxing. According to this arrangement the fibres may be oriented in a direction such that the friction coefficient will be lowest in the forward direction of movement. However, nothing is mentioned as to how the fibres can be ordered during production. The disclosed structure will provide a friction coefficient that is equal in the backward directions. Directionally oriented fibres admittedly will provide an anisotropic friction coefficient, but such a pattern is assumed to have little effect.
A satisfactory solution of the adherence to snow of non-wax plastic skis has previously been achieved by mechanically forming a plurality of indentations and hills in the ski base. The indentations each terminate in a steep edge to produce an accumulated effect resulting in a higher friction against than in the speed directions. The indentations, together with the flat, glide portions of the base, form a direction dependent pattern having as a result an anisotropic friction coefficient. A wellknown system of this type has a fish scale like structure. However, such systems have the general drawback that the three-dimensional pattern is relatively rapidly worn down. In addition all such systems produce scraping, vibration and an irritating "singing" noise in the ski track.